When placing a floor covering over concrete slabs, a contractor must first evaluate the concrete to ensure proper acclimation has been met. We often use two different methods for evaluating moisture in materials: a qualitative analysis and a quantitative analysis.
Qualitative vs Quantitative Analysis
Often times with a moisture meter we describe the scale as a qualitative number or a quantitative number. A qualitative number is often referred to when using a reference scale. A reference scale commonly has a range from 0-100 and should be used in terms of more moisture or less moisture. We always tell our customers that this scale should only be used as a comparative reading. Thus, one should take a reading in an area where there are no moisture problems and compare that reading to an unknown area where there could be a leak or more moisture.
Another good application is when a floor installer is preparing to drill holes so he can set his RH probes to do moisture tests using ASTM F2170. The floor installer wants to set the RH probes so that he can get a good representation of the concrete slab. He uses the Ligno DuoTec BW in pinless mode, on setting 25, and starts scanning the floor. His average reading is between 35 and 45, however he has found a few spots that are reading a little higher in the 55 range. Since the floor installer wants to make sure the flooring that he is installing stays as beautiful as the day he puts it in, he drills the holes where the moisture meter was reading 55. The meter has picked up more moisture in these areas however the number 55 is only a qualitative number it is not a percentage of moisture. The floor installer drills the holes and sets his sensor. 72 hours later he is ready to take some RH readings. He is getting readings that range from 65% -70%. This is a quantitative measurement taken using a RH% scale. From this we can analyze if the concrete slab is ready to have a floor covering installed. Using the manufactures specifications, he can ensure that the concrete slab is ready for his floor covering.
Floor installer taking a measurement on setting 25 in order to get a qualitative analysis of the concrete slab. Notice there is no unit of measurement on the reading.
The moisture in a slab sits 40% into the concrete slab. Only when a floor covering is placed on top of the slab will the moisture move to the surface and cause floor covering failure. Lignomat pinless moisture meters read up to 3/4″ into material, thus the information that you get from a pinless reading cannot alone determine if a concrete slab is ready for a floor covering. It is always recommended to use the ASTM F2170 test method for evaluating a concrete slab.